The present invention relates generally to induction heating and, particularly, to a system for inductively heating a work piece based on temperature feedback from a plurality of temperature feedback devices.
Induction heating is a method of heating that utilizes a varying magnetic field to heat a work piece. This varying magnetic field is produced by transmitting an alternating current through an induction heating device. A work piece located inside or in close proximity to the induction heating device is exposed to the varying magnetic field, inducing movement of electrons and causing a flow of eddy currents in the work piece. These eddy currents and resistance to current flow within the work piece cause the temperature of the work piece to rise. A varying magnetic field may be produced by transmitting an alternating current through the coil. Thus, the amount of heat induced in the work piece may be controlled by changing the magnetic field strength as a result of varying the amount of alternating current flowing through the induction heating device.
Certain induction heating systems utilize a temperature feedback device to control the heating of the work piece. A temperature feedback device provides a signal representative of the temperature of the work piece at a single, specific location on the work piece. For example, in some applications, a work piece is heated to a desired temperature, and a temperature feedback device informs the system when this desired temperature has been reached. As another example, it may be desired to heat the work piece at a defined rate of temperature increase. To effectuate such control, the temperature feedback device enables the system to control the amount of power provided to heat the work piece, causing portions of the work piece to increase in temperature at the desired rate.
Uniformity in heating is affected by the arrangement of the induction heating coil, and in some cases, depending on it's arrangement, may cause the induction heating systems to fail to heat the part uniformly. That is, various portions of a given work piece may be at different temperatures with respect to one another. Therefore, the temperature of the work piece where the single temperature feedback device is located may not represent the temperature of the work piece as a whole. As a result, the induction heating system may apply too much power or too little power to heat the work piece as desired. In many applications, such as post-weld stress relief, the entire work piece, or a desired portion of the work piece, must be appropriately heated to achieve the desired changes in the material properties of the work piece. If a portion of the work piece is not heated as desired, the desired changes in the material properties of the work piece may not be achieved. Therefore, a technique is needed to accurately measure the temperature of the work piece, utilizing several temperature feedback devices, and use the temperature measurement information to control the heating and cooling process to assure the process meets the desired heating profile.